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Today's Topics:
1. RE: two hosts with the same IP (Cuttler, Brian R (HEALTH))
----------------------------------------------------------------------
Message: 1
Date: Thu, 11 Jul 2024 13:17:59 +0000
From: "Cuttler, Brian R (HEALTH)" <brian.cutt...@health.ny.gov>
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Cc: "Hefner, Joseph (HEALTH)" <joseph.hef...@health.ny.gov>,
"Banavali, Nilesh K (HEALTH)" <nilesh.banav...@health.ny.gov>
Subject: RE: two hosts with the same IP
Message-ID:
<sa9pr09mb5679695b26f1824959167b28ba...@sa9pr09mb5679.namprd09.prod.outlook.com>
Content-Type: text/plain; charset="utf-8"
Sten,
Okay, pretty much what I?d thought, just never actually saw it before.
Reservations don?t protect the IP, the right answer here is to change to limits
on the dynamic pool and make sure my reservations are outside of the pool
I?m actually wondering why I have a reservation for this at all, that would
also have resolve the issue, and would have not caused the users an issue as
that subnet creates DDNS entries for us.
An entry like this?
boot.log.2:May 5 08:42:28 cedar dhcpd[863]: Remove host declaration suraj or
remove 10.50.148.212
I don?t routinely scan the dhcp logs, will maybe have to create a job to find
such messages and notify me.
Our static entries by and large are non-global, they are within the stanza but
that hasn?t been a problem, will look to make updates as time allows.
Joe, Nilesh, without CC?d the list, can you see any reason why we need a
reserved IP for Suraj?
Thanks,
Brian
From: dhcp-users <dhcp-users-boun...@lists.isc.org> On Behalf Of Sten Carlsen
Sent: Thursday, July 11, 2024 6:12 AM
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: Re: two hosts with the same IP
ATTENTION: This email came from an external source. Do not open attachments or
click on links from unknown senders or unexpected emails.
I believe there should be a specific warning in the log during startup.
-- Best regards
Sten Carlsen
No improvements come from shouting:
"MALE BOVINE MANURE!!!"
On 11 Jul 2024, at 02.23, Glenn Satchell
<glenn.satch...@uniq.com.au<mailto:glenn.satch...@uniq.com.au>> wrote:
?
hi Brian,
fixed-address assignments must be outside the pool of dynamic addresses to
prevent this situation.
If the fixed-address device is down, then the dynamic pool will see that as
free when it does a ping check and happily assign it. then when the
fixed-address device requests the ip it will always be assigned.
This is mentioned in the docs, dhcpd.conf man page i think.
regards,
Glenn
On 11 July 2024 6:12?am, "Cuttler, Brian R (HEALTH)"
<brian.cutt...@health.ny.gov<mailto:brian.cutt...@health.ny.gov>> wrote:
Running DHCP on Ubuntu 22.04.4 LTS
brian@cedar:/var/log$ DHCP -V
Internet Systems Consortium DHCP Server 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
Noted one system in particular was having network issues, ping to the IP was
good, ping from the IP was problematic, lots of dropping pings in about 25
second groups/blocks.
Also having trouble maintaining SSH connection into the host, and host keys
needed to be removed and reaccepted.
On a hunch I checked the dhcp log and found two different MAC addresses
receiving the same IP.
Jul 9 11:49:38 cedar DHCP[1019]: DHCPREQUEST for 10.50.148.212 from
d4:5d:64:08:91:7c via 10.50.148.1
Jul 9 11:49:38 cedar DHCP[1019]: DHCPACK on 10.50.148.212 to d4:5d:64:08:91:7c
via 10.50.148.1
Jul 10 15:04:17 cedar DHCP[1019]: DHCPREQUEST for 10.50.148.212 from
b0:7b:25:de:57:84 (berkeley) via 10.50.148.1
Jul 10 15:04:17 cedar DHCP[1019]: DHCPACK on 10.50.148.212 to b0:7b:25:de:57:84
(berkeley) via 10.50.148.1
Jul 10 15:05:16 cedar DHCP[1019]: DHCPDISCOVER from b0:7b:25:de:57:84
(berkeley) via 10.50.148.1
Jul 10 15:05:16 cedar DHCP[1019]: DHCPOFFER on 10.50.148.212 to
b0:7b:25:de:57:84 (berkeley) via 10.50.148.1
Jul 10 15:05:16 cedar DHCP[1019]: DHCPREQUEST for 10.50.148.212 (10.50.156.21)
from b0:7b:25:de:57:84 (berkeley) via 10.50.148.1
Jul 10 15:05:16 cedar DHCP[1019]: DHCPACK on 10.50.148.212 to b0:7b:25:de:57:84
(berkeley) via 10.50.148.1
I also note that one of the two hosts has a reservation.
host Suraj {
hardware ethernet d4:5d:64:08:91:7c;
fixed-address 10.50.148.212;
}
While I realize that a host with a reservation might be down, and another
machine could get the IP if its in the available pool I?m very surprised by the
behavior we were seeing.
If the non-received MAC received the IP wouldn?t the MAC with the reservation
still cause a ping test for availability to be performed?
I can?t think of any other way this could have happened.
Can anyone help me to understand what actually happened and how best to prevent
similar events in the future?
thanks in advance,
Brian
Brian Cuttler, System and Network Administration
Wadsworth Center, NYS Department of Health
Albany, NY 12201 POB 509
brian.cutt...@health.ny.gov<mailto:brian.cutt...@health.ny.gov>
518 486-1697
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